Refer to corresponding test files for usage examples and other guides

This MR updates the Usage Examples, Specializing, and Integration Guide documentation pages to inform about the possibility to download the source files from which the examples are included into the docs.

Can this MR be accepted?

• Documentation extended or updated as discussed in #186 (closed)
• No code changes (except tested includes)

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @

Related issues

#186 (closed) (not closing)